The above is true. Also, if you intend on prepping a lot, and you start out at pretty similar levels on the SAT and ACT, I recommend the SAT more. I find with my students that it is a bit more conducive to studying and prepping. Fortunately, though, the tests are pretty similar these days so if you study for one that will you a lot on the other.

That would be a very long post and there are many things written on CC about this topic. Still, here is some general advice.

Math: math is mainly about knowing the content and having a good mental database of problems. The former should come first. Make sure you know all the math on the test. Make sure that you are especially good at setting up and solving equations (probs the most important skill on the SAT, esp. on word problems). Then you should take a bunch of official, college board published practice tests. Go over your answers and don't give yourself the benefit of the doubt. Unless you clearly made a careless mistake, assume you need to go over the topics covered by the problem. Go over the topic, then do another practice test, etc. If you make a lot of careless mistakes, figure out how not to (e.g. write out more steps, get used to checking units on graphs, etc.). The more problems you do, the more familiar SAT style problems should be. Just make sure you know the content first (Khan Academy may be a good place to start). Otherwise, practice won't get you very far. The are other sources of good math problems that will help you on the SAT. I recommend the Ivy Global 6 practice tests book for math (the reading and writing in it is not as good). If you are a higher level student, the GRE math sections are also good to look into--the problems are generally more difficult, but don't require much more knowledge.

Writing: the writing section is mainly about knowing various grammar and composition rules. Learn these rules and get very comfortable at applying them, and you should be pretty good. Just know, that the correct answer is never arbitrary--there is always a good reason behind it (the same goes for reading). Try to figure out what the reason is. Take practice tests. Go over the ones you got wrong, but don't look at the explanations right away. Try to figure out the explanation before looking at it.

Reading: Take practice tests and analyze the ones you got wrong. Try to figure out why you got them wrong before looking at the explanations. Know that each correct answer will be supported by fairly direct evidence. If inferences need to be made to get to the answer, they will be pretty small inferences. As such, the main thing with reading is getting good at understanding the passage and finding evidence. I am a big proponent of reading slowly so you can answer quickly. Make sure you understand each sentence and each paragraph as you go along. Literally ask yourself at the end of each paragraph what the paragraph was about. If you don't understand something, try to figure it out. If the author has a thesis, make sure you understand it. Make sure you understand the general structure of the passage, i.e. the purpose of each paragraph with respect to the whole. If you have troubles understanding a sentence, there are a number of things you can do. Here are some of them. 1) Try to paraphrase small pieces of the sentence one by one to make the sentence more comprehensible.Then put these units together. 2) Try to replace pronouns with their referents (e.g. 'he; with 'George Washington'). 3) If a word doesn't make sense try to figure it out based on context. 4) Sometimes reading on to fill in gaps is necessary. If you do all these things well, the questions will be much easier. If you notice that there are a lot of unfamiliar words in the passages you see, you will need to study vocab (plenty of decent lists online) to improve your comprehension.

In general, I suggest taking practice tests untimed at first. First you have to understand how to answer all the questions. Then you worry about getting fast at them.